In 1958, a year before Ralph Keeling was born, his father set up an atmospheric monitoring station on a Hawaiian volcano. The average daily level of carbon dioxide in the atmosphere at the time stood at about 315 parts per million -- higher than it had been for most of human history, but not by much. Late last week, headlines around the globe reported that monitors on that same Hawaiian volcano were showing CO2 concentrations in the atmosphere had topped 400 ppm -- a level not seen for about 3 million years.

“It’s a psychological milestone,” says Keeling, who followed in the footsteps of his father, renowned geochemist Charles David Keeling, to become an atmospheric scientist at the Scripps Oceanographic Institution in California. I wrote about the younger Keeling continuing his father’s work for the Winter 2013 issue of OnEarth (see “Air Apparent”), and I spoke to him briefly this week to get his reaction to the atmospheric monitoring readout heard round the world.

“It’s like turning 50,” Keeling told me. “You don’t think that much about your age until you reach that milestone. You don’t think abut CO2 levels going up year by year -- it’s hardly newsworthy when levels go from 380 to 382 -- until you reach that landmark and realize the levels have gone up an awfully long ways.”

Keeling’s father meticulously documented that steady rise (which bears his name, the Keeling curve) over the course of half a century, providing indelible proof that the burning of fossil fuels is significantly altering the planet’s atmosphere. Carbon dioxide levels were approaching 340 ppm when Ralph, too, began taking atmospheric measurements in the early 1980s. And they were at 380 ppm when his father died in 2005 and Ralph took over the long-term monitoring program.

Even more important than the growing number of CO2 molecules in the air is the rate at which they have been increasing annually -- from 0.7 ppm per year in the late 1950s to 2.1 ppm per year during the last decade. At this pace, Keeling warns, we could hit 450 ppm -- a level widely regarded as what’s needed to keep global temperature rise to no more than 2°C above pre-industrial levels -- within two decades.

Despite last week’s headlines, it’s now not exactly clear when or if CO2 levels strayed across the 400 ppm threshold. Daily readings are based on preliminary data, which are often revised. After announcing a reading of 400.03 ppm last Thursday, scientists at the National Oceanic and Atmospheric Administration revised that measurement and now say it was actually 399.89 ppm. The preliminary data for yesterday, though, suggests a reading of 400.07 ppm.

Atmospheric concentrations of CO2 fluctuate. The levels rise and fall daily, as well as seasonally. The amount of carbon in the air peaks at this time of year, then starts dropping as the Northern Hemisphere’s plants and forests grow, leaf out, and draw carbon dioxide out of the air for the process of photosynthesis. Keeling compares the shifting carbon levels to waves lapping at a beach. “What we’re seeing are the first ripples up to the 400 level.” He predicts that by this time next year, the average monthly -- not just daily -- reading at Mauna Loa will be 400 ppm.

Change -- especially global change -- takes time. Day to day, it’s easy to overlook that fact. We don’t care much, or even really notice, if temperatures are a half-degree warmer this year than last, or if the sea is pushing a half-inch higher on shore. But landmarks like 400 ppm can turn our attention to the screw-turn of time and spotlight the incremental advance of change. At least that’s Keeling’s hope.

“This 400 ppm milestone," he says, "is an opportunity to get people to pay attention to what’s happening” -- while there's still a chance to make a difference.

Susan Freinkel is the author of Plastic: A Toxic Love Story and American Chestnut: The Life, Death, and Rebirth of a Perfect Tree. She has also written for the New York Times, Discover, Smithsonian, Mindful, and other publications.

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